1. Field of the Invention
The present invention relates to a photovoltaic device composed of a silicon-based non-single-crystal semiconductor material, and more particularly to a photovoltaic device having a high photoelectric conversion efficiency and high reliability.
2. Related Background Art
The photovoltaic device is a semiconductor device for converting optical energy such as solar light into electric energy. As the semiconductor material therefor, amorphous materials represented by amorphous silicon (a-Si:H) is attracting attention and is being intensively investigated because such materials are inexpensive, and enable preparation in a large area and in a thin film, a large freedom in composition and control of the electrical and optical characteristics over a wide range.
In the photovoltaic device composed of the above-mentioned amorphous material, an improvement in the photoelectric conversion efficiency is an important requirement.
As a method for meeting such requirement, the U.S. Pat. No. 2,949,498 proposes the use of so-called tandem cell formed by stacking a plurality of solar cells of a unit element structure. Such tandem cell is to improve the conversion efficiency by stacking the elements of different band gaps and efficiently absorbing the different portions of the spectrum of solar light. Such cell is so designed that, in comparison with the band gap of so-called top layer positioned at the light incident side of the stacked elements, the band gap of so-called bottom layer positioned under such top layer is narrower. There is also investigated a three-layer tandem cell (hereinafter, referred to as xe2x80x9ctriple cellxe2x80x9d) having so-called middle layer between the aforementioned top and bottom layers.
Also there is often adopted a configuration having a p-layer at the transparent electrode side, namely at the light incident side in order to facilitate collection of the positive holes having a shorter diffusion distance within the electron-positive hole pairs generated from the incident light, thereby improving the total light conversion efficiency, and also having a substantially intrinsic semiconductor (hereinafter represented as xe2x80x9ci-type layerxe2x80x9d) between the p-layer and an n-layer.
Also there can be achieved an improvement in the short circuit current (Jsc) by employing microcrystalline silicon in the p-layer at the light incident side, utilizing the high conductivity and the low absorption coefficient at the short wavelength region of microcrystalline silicon. Further, microcrystalline silicon, having a wider band gap in comparison with amorphous silicon, shows a higher efficiency for the addition of impurities and increases the internal electric field in the photovoltaic device. As a result, there are reported improvements in the open circuit voltage (Voc) and the photoelectric conversion efficiency (xe2x80x9cEnhancement of open circuit voltage in high efficiency amorphous silicon alloy solar cellsxe2x80x9d S. Guha, J. Yang, P. Nath and M. Hack: Appl. Phys. Lett., 49 (1986) 218).
In such photovoltaic devices, however, it is difficult to stably control the interfacial characteristics of the junction portion of the p- and n-layers, and the fluctuation in the junction state and the amount of impurities causes an increase in the serial resistance and a deterioration in the IV characteristics resulting therefrom, thereby leading to variations in the characteristics.
In consideration of the foregoing, the object of the present invention is to provide a photovoltaic device capable of stabilizing the control of the interface of the junction portion between the p- and n-layers and improving the interface characteristics, thereby achieving a high photoelectric conversion efficiency.
A photovoltaic device of the present invention is a photovoltaic device comprising a plurality of unit elements stacked mutually while forming p/n type junction, each of the unit elements having a pn or pin structure composed of a silicon-based non-single-crystal semiconductor material, wherein the nitrogen concentration has a maximum peak at the junction interface of the aforementioned p/n type junction, and the nitrogen concentration at the maximum peak (hereinafter, referred to as xe2x80x9cpeak nitrogen concentrationxe2x80x9d) is within a range from 1xc3x971018 to 1xc3x971020 atom/cm3.